CABL Forum - Update

Okay,

So we're starting to see a pattern with upstream MER(SNR) issues. The number one factor that has been causing this issue to pop up is the move from 16 QAM to 64 QAM on the upstream carrier. Its just like the upgrade from 128 QAM to 256 QAM all over again. To all you techs battling low upstream SNR, I can give you three pieces of advice:

1) sweep
2) uh, sweep
3) yup... sweep

Unfortunately, we've found that we couldn't do a rough sweep. We had to hit all the actives and then every other tap, re-referencing as we went to find any irregularities. It took some pretty cool detective work to fix the problems. Here's a step-by-step process:

1) print off a list of all modems in the node with low upstream SNR
2) print off a copy of the node map (if you have prints of certain sections, tape them together to form one map).
3) with a co-worker, have him give you streets and addresses. You highlight as he reads them off. (This is just like old-school days of determining the common point of outages).
4) find the common point. It should be limited to just a few spans of cable. highlight these spans.
5) get a test modem, a laptop, and an inverter.
5) go out into the field. start at the beginning of the common point and reference your forward sweep.
6) hit every other tap, re-referencing as you go. If you find ANYTHING out of the ordinary (i.e. obvious faults, no matter how small; larger min/max than expected; etc.) you will need to replace/temp the span, but first, go to step (7).
7) hook up modem to the tap that is fed by the suspect span and run a diagnostics to check upstream SNR.
8) replace or temp the span and re-check upstream SNR.

Step number 8 can be a very slippery situation. You might replace the span of cable and only gain 0.1 dB on the upstream SNR. However, I promise you that 0.1 you see at the front will translate to about 1.0 dB at the EOL where all your problems are undoubtedly occurring. Everyone's heard of the funnel effect. Low upstream SNR issues are the epitome of funnel effect issues, because it's always seen AFTER the bad span and more importantly, usually towards the end of the line. In other words, bad cable in the front of a subdivision can cause low upstream SNR 5 actives and 10 blocks away.

One more thing to avoid at all costs. We had this changed just the other day. If you have two nodes sharing the same port at the CMTS in the head end, do everything you can to separate the two, even if there's only 50 modems per node. We did this on Wednesday and every modem had an increase in upstream SNR of at least 5 dB. There is something about the combining effects causing the upstream noise floor to be higher.

Again, I'm new to the system tech world so I'm not sure if I'm giving any news you guys haven't yet heard. However, I feel its important to share the information. We all have to stick together right? Hope this helps.

- J_r0kk

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